Secondary (i.e., rechargeable) lithium ion batteries have been implemented as a power source into a wide variety of stationary and portable applications. Their structure and electrochemical reaction mechanism provide them with several desirable characteristics including a relatively high energy density, a relatively low internal resistance, a general non-appearance of any memory effect when compared to other types of rechargeable batteries, for example, nickel-cadmium batteries, and a low self-discharge rate. These characteristics have made lithium ion batteries the preferred mobile power source for portable consumer electronics such as laptop computers and cell phones. Larger-scale versions that interact with a multitude of interconnected systems have also been designed and manufactured by the automotive industry in an effort to improve vehicle fuel efficiency and reduce atmospheric pollution. The powertrains of hybrid electric vehicles (HEV) and extended range electric vehicles (EREV), for example, rely on the cooperative effort of lithium ion batteries and a hydrocarbon-fueled internal combustion engine to generate torque for vehicle propulsion.
A lithium ion battery generally contains one or more electrochemical battery cells that include a negative electrode, a positive electrode, and a porous polymeric separator sandwiched between the confronting inner face surfaces of the electrodes. The negative electrode generally includes a lithium host material that stores intercalated lithium at a relatively low electrochemical potential (relative to a lithium metal reference electrode). The positive electrode generally includes a lithium-based active material that stores intercalated lithium at a higher electrochemical potential than the lithium host material (relative to the same lithium metal reference electrode). The interadjacent porous separator includes opposed major surfaces that intimately contact the confronting inner face surfaces of the electrodes. A main function of the separator is to provide a porous and electrically insulative mechanical support barrier between the negative and positive electrodes to prevent a short-circuit in the cell. Each of the negative electrode, the positive electrode, and the separator is wetted with a liquid electrolyte solution that can communicate lithium ions. The liquid electrolyte solution is typically a lithium salt dissolved in a non-aqueous liquid solvent.
An interruptible external circuit electrically connects the negative electrode and the positive electrode to provide an electrical current path around the separator to electrochemically balance the migration of lithium ions. Metallic current collectors intimately associated with each electrode supply and distribute electrons to and from the external circuit depending on the operating state of the electrochemical batter cell. The external circuit can be coupled to an electrical load (during discharge) or an applied voltage from an external power source (during charging) through conventional electronic connectors and related circuitry. A voltage of approximately 2.5V to 4.3V is usually attained in each electrochemical battery cell during battery discharge. Greater overall battery power levels can be achieved, if necessary, by linking together a suitable number of similar electrochemical battery cells with their negative and positive electrodes connected in series or in parallel to corresponding common terminals. Current lithium ion batteries intended to be used in a vehicle powertrain typically include anywhere from 10 to 150 individual electrochemical battery cells. Several of these lithium ion batteries can be further connected in series or in parallel and packaged together to form a lithium ion battery pack that achieves a desired overall voltage and current capacity.
Conventionally, the porous polymeric separator has been composed of a polyolefin such as polyethylene and/or polypropylene. A number of fabrication methods have been developed for making a polyolefin separator with its intended porosity. The separator may be formed by a dry technique in which the polyolefin polymer is melted, extruded into a film, annealed, and then uniaxially stretched. The annealing and stretching phases produce and refine tightly ordered pores throughout the bulk interior of the resultant separator. The separator may also be formed by a wet technique in which the polyolefin polymer is mixed with a hydrocarbon or other low-molecular weight liquid substance. The mixture is then heated, melted, and formed into a film. Afterwards, the hydrocarbon or other low-molecular weight liquid substance is extracted. The extraction of the hydrocarbon or other low-molecular weight liquid substance induces pore formation throughout the resultant separator. Further refinement of the pores can be achieved by stretching or drawing the separator if desired.
But lifetime and performance declines are a concern for an electrochemical battery cell that includes a polyolefin separator. Exposure of the electrochemical cell to temperatures of 100° C. and above can cause the polyolefin separator to shrink, soften, and even melt if the temperature approaches 130° C. Such high temperatures can be attributed to charging-phase heat generation, ambient atmospheric temperature, or some other source. The temperature-initiated physical distortion of the polyolefin separator may ultimately permit direct electrical contact between the negative and positive electrodes and cause the electrochemical cell to short-circuit. Battery thermal runaway is also a possibility if the electrodes come into direct electrical contact with one another to an appreciable extent.
A variety of engineering polymers that exhibit better thermal stability than polypropylene and polyethylene have been investigated as candidates for a lithium ion battery separator. But the separator fabrication methods often used for polyolefin separators generally cannot provide these types of polymers with a sufficient porosity across their thickness. The porosity of the separator is not a physical property to be taken lightly as it impacts the performance and longevity of the electrochemical battery cell. A separator with insufficient porosity impedes lithium ion migration between the electrodes while a separator with too much porosity is susceptible lithium dendrite crossover from the negative electrode. A fabrication method that can reliably produce, from a variety of engineering polymers, a thermally stable polymer separator having a uniform, tortuous interconnected network of pores defined coextensively throughout the separator is therefore needed.